In America today, three major credit bureaus--TransUnion, Equifax and Experian--maintain detailed credit reports on most consumers 18 years and over.

This system has roots going back over 100 years. Credit scoring emerged later, when the Fair Isaac Corporation (FICO) launched it's first scoring system to predict consumer lending risk in the late 1950s. FICO partnered with its first credit bureau at the end of the 1980s, and 60 years after its launch there are almost too many credit scoring models to count. The score that a lender will use is likely to be different than any score you can access yourself, and the credit score itself is one of several key points of evaluation that can go into a lending decision. A credit score is a great indication of your credit health but should only be treated as a guide.

At Credit Karma, over 35 million members access their credit scores from two major bureaus for free. But it is important for our members, and any other American consumer looking at their score, to keep the following things in mind.

1) There is not "one" credit score.

Some people will try and tell you--incorrectly--that there is one true credit score when in actuality there are hundreds. FICO is the best-known credit scoring brand in America, but it has over 50 scores available. Credit score providers often tweak scoring models for different purposes. Further, each scoring model is applied in slightly different forms by the three major bureaus, making the number of available scores exponentially greater. For example, VantageScore is another credit scoring brand, established by the three major bureaus in 2006. VantageScore has three different models, each offered in slightly different forms through the three major bureaus.

Each of the major credit reporting bureaus could give you a different score, even if the scoring model applied is the same. Each bureau stores its information slightly differently, with information reported to the bureaus at different times. Your credit report is a unique thumbprint that can take into account some combination of over 200 factors. If one information point varies, the final score can differ.

Bottom line: Choose the credit score that you are going to track over time, and generally speaking, as that score changes, the others will, too. Pick your benchmark and stick with it.

2) No matter what your score is, the data comes from the same place.

Credit scoring models are essentially all slightly different equations made from the same set of information--the credit reports that each of the bureaus maintains on you. However many credit scoring models there are, there are only three major bureaus. The information on their credit reports is more important than the many fluctuations in your score. For a start, make sure that there are no errors on your credit report. A 2013 FTC study found that twenty-five percent of consumers identified an error on their credit report that might affect their scores. Your credit report is a document of how responsibly you've managed debt in the past. All credit scoring models look at factors like whether you pay your bills on time, how much of your available credit you're using (it's generally good to keep your card utilization rate under 30 percent), the length of your credit history and how frequently you're applying for new credit. If you focus on managing your credit responsibly, this will be evident on you credit report and your score will likely take care of itself.

3) You can get turned down even with excellent credit.

The harder truth to swallow is that even the best credit score might not get you the line of credit you want. Lenders have their own custom risk analysis models and may require you to have a job or earn a certain amount of money to qualify. They may balance your current debts against your current salary to calculate your debt burden. Some won't approve any applicant with a bankruptcy, missed payment or account in collection on their record, no matter how their credit score has recovered.

4) Despite all of this, there's always benefit in knowing your credit score.

With all of the scoring models on the market, it is unlikely that the credit score you look at will be the one your lender sees. But that's no reason not to stay up to date on your credit score and report. Your score is one of the fundamental metrics lenders use in evaluating your application--think of it as the first point of qualification into a lending decision-making process. For most credit products, you won't get approved without one that lenders view as acceptable. Knowing your credit score before you apply for any new credit helps you understand the range of interest rates you can expect and what products you might qualify for.

Different models don't vary that greatly, either. No matter what score your lender ultimately looks at, each credit scoring model adheres to similar guidelines to be truly predictive, using the same base set of data from credit bureaus and similar statistical procedures. The chances of a huge fluctuation are slim. A 2012 report from the Consumer Financial Protection Bureau looked at credit score information from 200,000 consumers to calculate the difference in scores between score providers. Comparing the different scoring models as offered by the three major credit bureaus, they found that only 1-3 percent of consumers had scores that differed by two or more credit categories. To combat these slight variances, you can shop around for credit offers to protect against the chance that one lender is looking at a credit score for you that differs a lot from the others. You can do this without impacting your credit score, too. If you're applying for a mortgage or auto loan, VantageScore treats all inquiries made within a two-week window as one hard pull, while FICO lumps all similar inquiries within 30 days together.